alrite lads Got an annoying fault with me focus and need some help!

Ive a 99 1800zetec. when driving along every so often (average of every 10 minutes) there it a small power surge at the same time the speedo needle drops to zero. If i am about to stop at traffic lights and i dis-engage the clutch the revs will virtually always drop to nothing then causing the car to stall and needs to be restarted. after a few minutes the speedo engages again along with another power surge. although this never seems 2 happen when on the motorway averaging 80mph or so. A mecanic friend of mine ran a Snap-On scanner in the ECU which showed no problems (although vehicle was stationary when tested!) any ideas??
cheers.